TUAW has learned that, with the release of iPhone OS 3.1 beta 2, tethering via the IPCC hack no longer works for AT&T customers. While iPhone OS 3.0 provides support for tethering, AT&T has yet to officially support it. As a workaround to obtain tethering (as well as MMS for some), some users have modified their carrier files (IPCC). As of 3.1 beta 2, this no longer works.
Those who need tethering may want to avoid the 3.1 update when it becomes available.